Process for producing porous alginate-based aerogels

ABSTRACT

The present invention relates to a process for preparing a porous material, at least comprising the steps of providing a mixture (I) comprising a water soluble polysaccharide, at least one compound suitable to react as cross-linker for the polysaccharide or to release a cross-linker for the polysaccharide, and water, and preparing a gel (A) comprising exposing mixture (I) to carbon dioxide at a pressure in the range of from 20 to 100 bar for a time sufficient to form a gel (A), and depressurizing the gel (A). Gel (A) subsequently is exposed to a water miscible solvent (L) to obtain a gel (B), which is dried. The invention further relates to the porous materials which can be obtained in this way and the use of the porous materials as thermal insulation material, for cosmetic applications, for biomedical applications or for pharmaceutical applications.

This is a divisional of U.S. application Ser. No. 15/312,541, filed Nov.18, 2016, which is the National Stage of International application no.PCT/EP2015/060872, filed May 18, 2015, which claims priority to Europeanpatent application no. 14168810.1, filed May 19, 2014, of which all ofthe disclosures are incorporated herein by reference in theirentireties.

The present invention relates to a process for preparing a porousmaterial, at least comprising the steps of providing a mixture (I)comprising a water soluble polysaccharide, at least one compoundsuitable to react as cross-linker for the polysaccharide or to release across-linker for the polysaccharide, and water, and preparing a gel (A)comprising exposing mixture (I) to carbon dioxide at a pressure in therange of from 20 to 100 bar for a time sufficient to form a gel (A), anddepressurizing the gel (A). Gel (A) subsequently is exposed to a watermiscible solvent (L) to obtain a gel (B), which is dried. The inventionfurther relates to the porous materials which can be obtained in thisway and the use of the porous materials as thermal insulation material,for cosmetic applications, for biomedical applications or forpharmaceutical applications.

Porous materials, for example polymer foams, having pores in the sizerange of a few microns or significantly below and a high porosity of atleast 70% are particularly good thermal insulators on the basis oftheoretical considerations.

Such porous materials having a small average pore diameter can be, forexample, in the form of organic aerogels or xerogels which are producedwith a sol-gel process and subsequent drying. In the sol-gel process, asol based on a reactive organic gel precursor is first produced and thesol is then gelled by means of a crosslinking reaction to form a gel. Toobtain a porous material, for example an aerogel, from the gel, theliquid has to be removed. This step will hereinafter be referred to asdrying in the interests of simplicity.

The present invention relates to a process for the manufacture ofpolysaccharide-containing porous materials, as well as to the porousmaterial as such and their use. In particular, the invention relates toa process for the manufacture of alginate-containing porous materials.It is for example known that alkali alginates such as sodium alginateare water-soluble, whereas earth-alkaline alginates such as calciumalginates are insoluble in water. Thus, gels can be prepared from watersoluble polysaccharides, in particular natural polysaccharides such asalginates. However, for example in case of alginates, if manufacture ofthicker layers is intended, difficulties arise from the fact that thehomogeneous incorporation of free calcium ions into a sodium alginatesolution is made difficult by the large increase of the solution'sviscosity, so that disjointed calcium alginate agglomerates are theresult instead of uniform products.

To overcome this problem, U.S. Pat. No. 5,718,916 suggests, for example,to add a water-soluble complexing agent such as sodium citrate to theaqueous solution of the alginate composition. If, for example, an easilysoluble calcium salt such as calcium chloride is subsequently added, theimmediate precipitation of calcium alginate is prevented by the presenceof the complexing agent, which is supposed to prevent the formation ofinsoluble calcium alginate globules in the product. However, theexamples of U.S. Pat. No. 5,718,916 are based of the scale of a fewmillimeters.

The manufacture of small-scale alginate sponges for oral use by addingsoluble calcium salt (calcium gluconate) to a sodium alginate solutionis described in WO 01/17377. However, for the reasons already mentionedabove (no homogeneous incorporation of the calcium ions), this processis also not suitable for the manufacture of large-format alginatesponges. The application of active substances suggested therein is alsomade difficult due to the inhomogeneities that arise.

A process for the manufacture of polysaccharide foams, in particularbased on an alginate, is known from WO 94/00512. According to oneembodiment, WO 94/00512 also discloses a variant in which an insolublecarbonate or bi-carbonate salt are dispersed in the foamedpolysaccharide by polyvalent metal cations and the foam subsequentlytreated with a strong acid in order to release carbon dioxide and tocrosslink, by the cations that form, the polysaccharide while adimensionally stable foam structure is formed. According to the printedpublication, foams of a thickness of up to 5 mm can be stabilized inthis manner. However, the formation of gases during manufacture leads todifficulties in controlling the size of the pores and to greatinhomogeneities in the foam.

Another process for the manufacture of alginate sponges is known fromU.S. Pat. No. 3,653,383. Here, calcium alginate is at first producedfrom alginic acid and calcium carbonate, the calcium alginate formed isthen ground, and the resulting gel is subjected to freeze-drying.Relatively large-format sponge-like materials can be produced in thismanner, however, the products obtained disintegrate relatively quicklyin water. Thus, the alginate sponges—in particular when cut into thinlayers—have a wet-strength, in particular with regard to wet breakingstrength, which is insufficient for cosmetic or medical pads.

Also in the scientific literature, gelation processes induced bypressurized CO₂ in alginate-based systems are disclosed. Partap et al.(2006, “Supercritical Carbon Dioxide in Water” Emulsion-TemplatedSynthesis of Porous Calcium Alginate Hydrogels. Advanced Materials 18,501-504) showed that sodium alginate/Ca-Citrate mixture (alginateconcentrations of 8% w/v) undergoes gelation being dispersed in sc-CO₂in presence of surfactant at 100 bar, 50° C. The Ca-Citrate is preformedfrom Na-Citrate and CaCO₃ and then reacted with Na-Alginate under theabove mentioned conditions to release Ca-Ions. In the course of theCa-Alginate formation Na-Citrate is formed as a byproduct, which resultsin reversible hydrogels (destabilized within 48 h), which have to beadditionally reinforced by immersion in a solution with free calciumions.

Draget et al. (1990, Homogeneous alginate gels: a technical approach.Carbohydrate Polymers 14, 159-178) used carbon dioxide at 1 bar to gel asodium alginate/CaCO₃ mixture (1 wt % of alginate). They found thatgelation time is extremely dependent on gel depth. A slice of 8 mmthickness had not gelled completely within 48 h.

However, due to the fast gelation at low alginate content the processesas disclosed in the state of the art do not allow to produce stable andhomogeneous hydrogels. Hydrogels and thus the resulting aerogelsproduced by existing methods are dense and have never been appreciatedfor thermal insulation purposes. Furthermore, the gelation with pHreducers leads to organic byproducts entrapped in the hydrogels and thusthe resulting aerogels.

It was therefore an object of the invention to avoid the abovementioneddisadvantages. It was a further object of the present invention toprovide stable gels using a simple process. In particular, a porousmaterial based on a polysaccharide, in particular based on an alginatewhich does not have the abovementioned disadvantages, or has them to areduced extent, should be provided. It was a further object of thepresent invention to provide porous materials which are suitable asthermal insulation material or for cosmetic or pharmaceuticalapplications. The porous materials should have a low thermalconductivity in the ventilated state, i.e. at atmospheric pressure.Furthermore, it was an object of the present invention to provide aprocess for preparing homogeneous gels from water solublepolysaccharides, in particular natural water soluble polysaccharidessuch as for example alginates.

According to the present invention, this object is solved by a processfor preparing a porous material, at least comprising the steps of:

a) providing a mixture (I) comprising

-   -   (i) a water soluble polysaccharide,    -   (ii) at least one compound suitable to react as cross-linker for        the polysaccharide or to release a cross-linker for the        polysaccharide,    -   (iii) water,        b) preparing a gel (A) comprising steps b1) and b2)    -   b1) exposing mixture (I) to carbon dioxide at a pressure in the        range of from 20 to 100 bar for a time sufficient to form a gel        (A), and    -   b2) depressurizing the gel (A),        c) exposing the gel (A) obtained in step b) to a water miscible        solvent (L) to obtain a gel (B),        d) drying of the gel (B) obtained in step c).

According to the present invention, water soluble polysaccharides areused to form gels. Among them, the use of natural polysaccharides and/ortheir derivatives are especially attractive because of their stability,availability, renewability and low toxicity.

For the purposes of the present invention, a gel is a crosslinked systembased on a polymer which is present in contact with a liquid (known assolvogel or lyogel), or with water as liquid (aquagel or hydrogel).Here, the polymer phase forms a continuous three-dimensional network.

In the context of the present invention, water soluble means that thesolubility in water is sufficient to form a solution which can be usedfor preparing a gel.

According to the present invention, a gel is formed from the watersoluble polysaccharide and a suitable cross-linker. The polysaccharideused for the process of the present invention has to be suitable to forma gel with a cross-linker, in particular has to have suitable functionalgroups.

Natural polysaccharides such as agar, alginate, carrageenan, cellulose,hyaluronic acid, pectin, starch, and xanthan gum as well assemi-synthetic polysaccharides such as modified cellulose, chitin andchitosan are particularly preferred.

It has surprisingly been found that the claimed method allows to producebiopolymer aerogels, i.e. polysaccharide based aerogels, with low solidcontent for thermal insulation with thermal conductivity in the range offrom 10 to 30 mW/m·K, preferably from 15 to 25 mW/m·K, particularly from18 to 20 mW/m·K. Properties of the aerogels can be customized byadjusting the reaction conditions at the stage of the formation of thehydrogel (gel (A)), or during solvent exchange as well as in the dryingstep. According to the present invention, it is possible to influencethe properties of the hydrogels and/or aerogels by varying the ratio ofthe components, as well as by pressure control and also by introducing awide range of organic and inorganic materials in the gel matrix. Bothmesoporous and macroporous (foam-like) aerogels can be produced by theprocess according to the present invention.

The combination of process steps according to the present inventionleads to an simplified process resulting in improved materials with highquality. According to the present invention, a hydrogel is formed whichis subsequently subjected to a solvent exchanges, in particular underpressure resulting in a lyogel. The lyogel is subjected to furthersteps. All steps of the process according to the present invention canbe carried out in an autoclave thus making the overall process simpleand efficient. Steps which involve manual handling can be avoided.

According to a preferred embodiment of the present invention, step c) iscarried out at elevated pressure. In the process of the presentinvention, step c) can be carried out at elevated pressure in the sameapparatus as the other process steps.

In particular the solvent exchange under pressure results in a furtherimproved process. The exchange of water and solvent is quicker. It hassurprisingly been found that this effect is even more pronounced forthicker samples making the process of the present invention suitable toprepare larger aerogel blocks.

According to the present invention, the water soluble polysaccharidepreferably is selected from the group consisting of agar, alginate,carrageenan, cellulose, hyaluronic acid, pectin, starch, xanthan gum,modified cellulose, chitin and chitosan.

According to a further embodiment, the present invention also relates toa process as described above, wherein the water soluble polysaccharideis an alginate.

The present invention thus also relates to a process for preparing aporous material, at least comprising the steps of:

a) providing a mixture (I) comprising

-   -   (i) a water soluble alginate,    -   (ii) at least one compound suitable to react as cross-linker for        the alginate or to release a cross-linker for the alginate,    -   (iii) water,        b) preparing a gel (A) comprising steps b1) and b2)    -   b1) exposing mixture (I) to carbon dioxide at a pressure in the        range of from 20 to 100 bar for a time sufficient to form a gel        (A), and    -   b2) depressurizing the gel (A),        c) exposing the gel (A) obtained in step b) to a water miscible        solvent (L) to obtain a gel (B),        d) drying of the gel (B) obtained in step c).

The process according to the present invention allows to producehomogeneous and stable hydrogels at low polysaccharide concentration,for example low alginate concentration, which result in aerogels whichare appropriate for thermal insulation with thermal conductivity in therange of from 10 to 30 mW/m·K, preferably from 15 to 25 mW/m·K,particularly from 18 to 20 mW/m·K.

Furthermore, a wide range of organic and inorganic materials can beentrapped, i.e. physically or co-gelled in the polysaccharide matrix,e.g. in the alginate matrix to achieve special properties. Furthermore,there are no organic byproducts associated with the process.

Suitable water soluble polysaccharides for use in mixture (I) accordingto step a) are in principle known to the person skilled in the art.Suitable salts are for example alkali salts such as sodium and potassiumsalts.

In case alginates are used, the water-soluble alginates preferably usedin step a) preferably are alkali metal alginates such as alginates ofsodium, or potassium. The underlying alginic acid is a natural acidpolysaccharide primarily extracted from so-called brown algae(Phaecophyceae) with a high molecular weight between 30,000 and 200,000,which contains chains formed from D-mannuronic acid and L-guluronicacid. The degree of polymerization changes depending on the kind of algaused for extraction, on the season during which the algae werecollected, the geographic origin of the algae as well as the age of theplants. The main kinds of brown algae from which alginic acid isobtained, are, for example Macrocystis pyrifera, Laminaria cloustoni,Laminaria hyperborea, Laminaria flexicaulis, Laminaria digitata,Ascophyllum nodosum and Fucus serratus. However, alginic acid or alkalialginates can also be obtained microbiologically, for example byfermentation with Pseudomonas aeruginosa or mutants of Pseudomonasputida, Pseudomonas fluorescens or Pseudomonas mendocina, see. e.g.EP-A-251905 and the entra regarding “alginic acid” in Rompp ChemieLexikon “Naturstoffe” (Encyclopedia of Natural Products) published byThieme Verlag, 1997.

In principle, all available water soluble polysaccharides which aresuitable to form a gel with a cross-linker can be used in the context ofthe present invention. The molecular weight of the polysaccharide andthe particle size may vary in wide ranges.

In the process according to the present invention, a mixture (I) isprovided according to step a). Mixture (I) comprises the water solublepolysaccharide, at least one compound suitable to react as cross-linkerfor the polysaccharide or to release a cross-linker for thepolysaccharide (as compound (ii)) and water.

According to the invention, in case alginates are used, alginates withan average particle size of up to about 0.2 mm and a viscosity inaqueous solution of from 300 to 800 mPas (1% solution, pH 7, 20° C.) arepreferred. According to the invention, sodium alginate is particularlypreferred. The aqueous solution of the water-soluble alginate used instep a) preferably has such a concentration, that, in mixture (I) aconcentration is formed of 0.2 to 3 wt %, more preferably 0.3 wt % to2.5 wt %, and still more preferably 0.4 wt % to 1.2 wt % of alginate inrelation to the amount of water used. The solution can be prepared bysuspending the desired amount of alginate in, e.g., distilled water.

According to the present invention, at least one compound suitable toreact as cross-linker for the polysaccharide or to release across-linker for the polysaccharide is present in mixture (I) ascompound (ii). This compound is preferably suspended or dispersed in theaqueous solution

The at least one compound suitable to react as cross-linker for thepolysaccharide or to release a cross-linker for the polysaccharide canbe a salt of a polyvalent metal ion. According to the present invention,preferably one or more salts of a polyvalent metal ion with amultidentate complexing anion are comprised in mixture (I).

According to the present invention, polyvalent metal ions are suitablewhich form poorly soluble compounds with the water solublepolysaccharide, in particular the alginate, used, i.e. which act ascross-linking metal ions. Such polyvalent metal ions include, forexample, alkaline earth metal ions and transition metal ions which formpoorly soluble compounds with polysaccharides, in particular alginates.Alkaline earth metal ions, such as magnesium or calcium are preferred.Calcium is particularly preferred. Calcium salts are particularlypreferred according to the invention for they are physiologically and,particularly, cosmetically acceptable and have a strong cross-linkingand/or gelation effect compared to alginates. In addition, e.g.beryllium, barium, strontium, zinc, cobalt, nickel, copper, manganese,iron, chromium, vanadium, titanium, zirconium, cadmium, aluminum canalso be used.

The polyvalent metal ions preferably are added in the form of theirpoorly soluble salts. In principle, the corresponding anions can beselected arbitrarily, however, in water, they must form poorly solublesalts with the polyvalent metal ions or cations. Preferably, carbonatesand hydroxy carbonates, such as cobalt carbonate, nickel carbonate, zinccarbonate, copper carbonate and others can be utilized, preferablytogether with or instead of calcium carbonate.

The amount of the poorly soluble salt of the polyvalent metal ion isselected, so that the concentration of the salt in the resultingsolution preferably is between about 0.1 and 500 mmol/litre, whereby, inthis case, the total amount of the salt in relation to the volume of thesolution is meant, even if the salt does not dissolve completely.

The amount of the added poorly soluble salt of the polyvalent metal ionin relation to the amount of the water soluble polysaccharide, inparticular alginate is preferably selected so that the molar ratio ofthe polysaccharide, for example the alginate, to the poorly soluble ofthe polyvalent metal ion is between 0.001 and 1.

According to a further embodiment, the present invention also relates toa process as described above, wherein the compound suitable to react ascross-linker for the polysaccharide or to release a cross-linker for thepolysaccharide is selected from the group consisting of carbonates andhydroxy carbonates. According to a further embodiment, the presentinvention also relates to a process as described above, wherein thecompound suitable to react as cross-linker for the alginate or torelease a cross-linker for the alginate is selected from the groupconsisting of carbonates and hydroxy carbonates.

According to a further embodiment, the present invention also relates toa process as described above, wherein mixture (I) comprises calciumcarbonate as compound (ii).

According to the present invention, mixture (I) can also comprisefurther compounds.

The mixture (I) provided in step (a) can also comprise further salts, inparticular such salts that do not form gels, and customary auxiliariesknown to those skilled in the art as further constituents. Mention maybe made by way of example of surface-active substances, flameretardants, nucleating agents, oxidation stabilizers, lubricants andmold release agents, dyes and pigments, stabilizers, e.g. againsthydrolysis, light, heat or discoloration, inorganic and/or organicfillers, reinforcing materials and biocides.

Further information regarding the abovementioned auxiliaries andadditives may be found in the literature, e.g. in Plastics AdditiveHandbook, 5th edition, H. Zweifel, ed. Hanser Publishers, Munich, 2001.

Furthermore, the mixture (I) can comprise cosmetic or medical activesubstances.

According to a further embodiment, the present invention thus alsorelates to a process as described above, wherein a water insoluble solid(S) is added to mixture (I).

According to a further embodiment, the present invention also relates toa process as described above, wherein a compound (C) is added to mixture(I) in step a) which is suitable to form a gel.

For example suitable organic and/or inorganic precursors suitable toproduce hybrid gels can be dispersed in mixture (I) as compound (C).Particularly suitable are further natural or synthetichydrocolloid-forming polymers. Thus, according to a further embodiment,the present invention thus also relates to a process as described above,wherein compound (C) is selected from the group consisting of naturaland synthetic hydrocolloid-forming polymers.

Further natural or synthetic hydrocolloid-forming polymers include(partially) water-soluble, natural or synthetic polymers that form gelsor viscous solutions in aqueous systems. They are expediently selectedfrom further natural polysaccharides, synthetically modified derivativesthereof or synthetic polymers. Further polysaccharides include e.g.homoglycans or heteroglycans such as, for example, carrageenan, pectins,tragacanth, guar gum, carob-bean gum, agar, gum arabic, xanthan gum,natural and modified starches, dextrans, dextrin, maltodextrins,chitosan, glucans, such as beta-1,3-glucan, beta-1,4-glucan, cellulose,mucopolysaccharides, such as, in particular hyaluronic acid. Syntheticpolymers include cellulose ethers, polyvinyl alcohol, polyvinylpyrrolidone, synthetic cellulose derivatives, such as methylcellulose,carboxycellulose, carboxymethylcellulose, in particular sodiumcarboxymethycellulose, cellulose esters, celluloses ethers such ashydroxypropylcellulose, polyacrylic acid, polymethacrylic acid,poly(methyl methacrylate) (PMMA), polymethacrylate (PMA), polyethyleneglycols etc. Mixtures of these polymers may also be used.

Moreover, one or more auxiliary substances might be comprised in mixture(I) according to the present invention. Auxiliary substances include:fillers, pH-adjustment agents, such as buffering substances,stabilizers, co-solvents, pharmaceutically and cosmetically conventionalor other dyestuffs and pigments, preservatives, plasticizers, lubricantsand slip additives.

According to step b) of the present invention, a gel (A) is prepared.Step b) comprises the steps b1) and b2). According to step b1), mixture(I) is exposed to carbon dioxide at a pressure in the range of from 20to 100 bar for a time sufficient to form a gel (A), and according tostep b2), the gel (A) is depressurized.

According to the present invention, step b1) is carried out at apressure in the range from 20 to 100 bar, preferably in a range of from30 to 80 bar, in particular in a range of from 40 to 60 bar, morepreferably in a range of from 45 to 55 bar. The pressure might beapplied for the duration of step b1). The pressure might also be variedover the duration of step b1).

According to step b), mixture (I) is exposed to pressurized carbondioxide at a suitable temperature preferably for several hours followedby depressurization to ambient pressure. Gel (A) is formed as a resultof the pressurization/depressurization process.

By exposing mixture (I) to carbon dioxide according to step b1), the pHvalue of the mixture (I) might be altered. Carbon dioxide, beingdissolved in aqueous media, acts as a weak acid and thus the formationof cations from insoluble carbonates initiating the crosslinkingreaction might be influenced. Adjustment of pressure, in particularcarbon dioxide pressure, and temperature thus allows to influence thegel structure.

According to a further embodiment, the present invention also relates toa process as described above, wherein the depressurization is carriedout at a rate in the range of from 0.05 bar/min to 5 bar/min, inparticular in the range of from 0.1 bar/min to 1 bar/min, more preferredin the range of from 0.5 bar/min to 1 bar/min.

In step b) of the process of the invention, the gel is usually formed byallowing to rest, e.g. by simply allowing the container, reaction vesselor reactor in which the mixture is present (hereinafter referred to asgelling apparatus) to stand. The mixture is preferably no longer stirredor mixed during gelling (gel formation) because this could hinderformation of the gel.

Gelling is known per se to a person skilled in the art and is described,for example, in WO 2009/027310 on page 21, line 19 to page 23, line 13.

Preferably, temperature and pressure in step b), in particular step b1)and b2), are adjusted to conditions under which carbon dioxide isgaseous. A suitable temperature might be in the range of from 10 to 40°C., preferable in the range of from 15 to 35° C. According to a furtherembodiment, the present invention also relates to a process as describedabove, wherein step b) is carried out at a temperature in the range offrom 10 to 40° C.

According to a further embodiment, the present invention also relates toa process as described above, wherein carbon dioxide is gaseous underthe reaction conditions in step b1).

Varying depressurization rate and thickness of the initial suspension,hydrogels can be produced either homogeneous (as assessed visually andby scanning electron microscopy) or with shell-like voids in themillimeter range. Thus, macroporosity can be introduced within thegelation step.

The rate of formation of the insoluble gel, in particular the alginate,and thus the flowability or pourability of the alginate solution orsuspension, can be controlled very exactly and easily by choosing asuitable temperature and/or pressure. Furthermore, a temperaturegradient and/or a pressure gradient might be applied.

Gel (A) obtained in step b) is a gel comprising water, i.e. a hydrogel.According to the present invention gel (A) obtained in step b) isexposed to a water miscible solvent (L) to obtain a gel (B) in step c)of the process of the present invention.

However, it is also possible to use the hydrogel (A) obtained as anintermediate of the process as disclosed above as such. Manyapplications for hydrogels are known. The hydrogel (A) is particularlyhomogenous, and thick stable layers can be prepared according to thepresent invention.

According to the present invention, a water miscible solvent (L) is usedin step c). In the context of the present invention, water misciblemeans that the solvent is at least partially miscible with water inorder to allow an exchange of solvent in the gel.

Solvent exchange is carried out either by soaking the gel directly inthe new solvent (one-step) or by following a sequential soaking(multi-step) in different water-to-new solvent mixtures with increasingcontent in the new solvent after a certain time (exchange frequency) inthe previous soaking step (Robitzer et al., 2008, Langmuir, 24(21),12547-12552). The solvent chosen for water replacement must satisfy therequirements of not dissolving the gel structure, being completelysoluble with the solvent which precedes them (water) and preferably alsoaccepted for manufacturing of pharmaceuticals.

The solvent (L) can in principle be any suitable compound or mixture ofa plurality of compounds, which meets the above requirements with thesolvent (L) being liquid under the temperature and pressure conditionsof step c).

According to the present invention, step c) might be carried out atambient pressure. However, it is also possible to carry out step c) atelevated pressure. Preferably, step c) is carried out at a pressureabove 10 bar, in particular at a pressure below 150 bar.

For example, step c) might be carried out at a pressure in the range offrom 10 to 150 bar, preferably in the range of from 10 to 100 bar, morepreferably in a range of from 20 to 80 bar, in particular in a range offrom 30 to 70 bar, more preferably in a range of from 40 to 50 bar.Furthermore, it is possible that step c) is carried out at a pressure inthe range of from 30 to 150 bar, preferably in the range of from 30 to120 bar, more preferably in a range of from 40 to 90 bar, in particularin a range of from 45 to 65 bar. The pressure might be applied for theduration of step c) or only for a short period.

Thus, the present invention according to a further embodiment alsorelates to a process as described above, wherein step c) is carried outat a pressure in the range of from 10 to 100 bar.

Furthermore, step c) might be carried out at elevated temperature, forexample a temperature in the range of from 10 to 80° C., preferably inthe range of from 15 to 70° C., more preferably in a range of from 20 to60° C. The temperature might be applied for the duration of step c) oronly for a short period.

According to a further embodiment of the present invention, step c) iscarried out at elevated pressure and elevated temperature, for exampleat a pressure in the range of from 10 to 150 bar and a temperature inthe range of from 10 to 80° C.

Possible solvents (L) are, for example, ketones, aldehydes, alkylalkanoates, organic carbonates, amides such as formamide andN-methylpyrollidone, sulfoxides such as dimethyl sulfoxide, aliphaticand cycloaliphatic halogenated hydrocarbons, halogenated aromaticcompounds and fluorine-containing ethers. Mixtures of two or more of theabovementioned compounds are likewise possible.

Further possibilities as solvents (L) are acetals, in particulardiethoxymethane, dimethoxymethane and 1,3-dioxolane.

Dialkyl ethers and cyclic ethers are likewise suitable as solvent (L).Preferred dialkyl ethers are, in particular, those having from 2 to 6carbon atoms, in particular methyl ethyl ether, diethyl ether, methylpropyl ether, methyl isopropyl ether, propyl ethyl ether, ethylisopropyl ether, dipropyl ether, propyl isopropyl ether, diisopropylether, methyl butyl ether, methyl isobutyl ether, methyl t-butyl ether,ethyl n-butyl ether, ethyl isobutyl ether and ethyl t-butyl ether.Preferred cyclic ethers are, in particular, tetrahydrofuran, dioxane andtetrahydropyran.

Aldehydes and/or ketones are particularly preferred as solvent (L).Aldehydes or ketones suitable as solvent (L) are, in particular, thosecorresponding to the general formula R²—(CO)—R¹, where R¹ and R² areeach hydrogen or an alkyl group having 1, 2, 3, 4, 5, 6 or 7 carbonatoms. Suitable aldehydes or ketones are, in particular, acetaldehyde,propionaldehyde, n-butyraldehyde, isobutyraldehyde,2-ethylbutyraldehyde, valeraldehyde, isopentaldehyde,2-methylpentaldehyde, 2-ethylhexaldehyde, acrolein, methacrolein,crotonaldehyde, furfural, acrolein dimer, methacrolein dimer,1,2,3,6-tetrahydrobenzaldehyde, 6-methyl-3-cyclohexenaldehyde,cyanoacetaldehyde, ethyl glyoxylate, benzaldehyde, acetone, diethylketone, methyl ethyl ketone, methyl isobutyl ketone, methyl n-butylketone, methyl pentylketone, dipropyl ketone, ethyl isopropyl ketone,ethyl butyl ketone, diisobutylketone, 5-methyl-2-acetyl furan,2-acetylfuran, 2-methoxy-4-methylpentan-2-one, 5-methylheptan-3-one,octanone, cyclohexanone, cyclopentanone, and acetophenone. Theabovementioned aldehydes and ketones can also be used in the form ofmixtures. Ketones and aldehydes having alkyl groups having up to 3carbon atoms per substituent are preferred as solvent (L).

Further preferred solvents are alkyl alkanoates, in particular methylformate, methyl acetate, ethyl formate, isopropyl acetate, butylacetate, ethyl acetate, glycerine triacetate and ethyl acetoacetate.Preferred halogenated solvents are described in WO 00/24799, page 4,line 12 to page 5, line 4.

Further suitable solvents (L) are organic carbonates such as for exampledimethyl carbonate, ethylene carbonate, propylene carbonate or butylenecarbonate.

In many cases, particularly suitable solvents (L) are obtained by usingtwo or more completely miscible compounds selected from the abovementioned solvents.

According to the present invention, a solvent (L) is used. The solvent(L) can also be a mixture of two or more solvents, for example three orfour solvents. Suitable solvents are for example mixtures of two or moreketones, for example mixtures of acetone and diethyl ketone, mixtures ofacetone and methyl ethyl ketone or mixtures of diethyl ketone and methylethyl ketone.

According to a further embodiment, the present invention therefore alsorelates to a process as described above, wherein the solvent (L) isselected from the group consisting of ethers, esters, alcohols, ketones,aldehydes, alkyl alkanoates, organic carbonates, amides, sulfoxides,aliphatic and cycloaliphatic halogenated hydrocarbons, halogenatedaromatic compounds and fluorine-containing ethers.

According to a further embodiment, the present invention also relates toa process as described above, wherein the solvent used in step c) isselected from the group consisting of C1 to C6 alcohols and C1 to C6ketones and mixtures thereof. Particularly suitable are alcohols such asmethanol, ethanol and isopropanol and ketones such as acetone, andmethyl ethyl ketone.

The solvent exchange according to step b) might be carried out in onestep or in multiple steps with varying concentration of the solvent.According to a preferred embodiment, gels (A) are successively immersedin ethanol/water mixtures with concentrations of for example 30, 60, 90and 100 wt % for 2 to 12 h in each.

In step c), gel (B) is obtained. According to step d) of the process ofthe present invention, gel (B) obtained in step c) is dried.

Drying in step (d) takes place in a known manner. Drying undersupercritical conditions is preferred, preferably after replacement ofthe solvent by CO₂ or other solvents suitable for the purposes ofsupercritical drying. Such drying is known per se to a person skilled inthe art. Supercritical conditions characterize a temperature and apressure at which CO₂ or any solvent used for removal of the gelationsolvent is present in the supercritical state. In this way, shrinkage ofthe gel body on removal of the solvent can be reduced.

However, in view of the simple process conditions, preference is givento drying the gels obtained by conversion of the liquid comprised in thegel into the gaseous state at a temperature and a pressure below thecritical temperature and the critical pressure of the liquid comprisedin the gel.

According to one embodiment, the drying of the gel obtained ispreferably carried out by converting the solvent (L) into the gaseousstate at a temperature and a pressure below the critical temperature andthe critical pressure of the solvent (L). Accordingly, drying ispreferably carried out by removing the solvent (L) which was present inthe reaction without prior replacement by a further solvent.

Such methods are likewise known to those skilled in the art and aredescribed in WO 2009/027310 on page 26, line 22 to page 28, line 36.

According to a further embodiment, the present invention is directed tothe process for preparing a porous material as disclosed above, whereinthe drying according to step d) is carried out by converting the liquidcomprised in the gel into the gaseous state at a temperature and apressure below the critical temperature and the critical pressure of theliquid comprised in the gel.

According to a further embodiment, the present invention is directed tothe process for preparing a porous material as disclosed above, whereinthe drying according to step d) is carried out under supercriticalconditions.

The process of the present can also comprise further steps such as forexample a shaping step.

According to the present invention, the porous material can for examplebe cut into shape after step d) of the process. According to the presentinvention it is also possible to form the gel in a defined shape.

According to the present invention, it is also possible to reduce thegel to smaller gel particles with a diameter in the range of from 10 to500 μm which then can be dried as set out above. According to thisembodiment of the process of the present invention, a porous material,in particular an aerogel, in the form of small particles is obtained.

According to the present invention, the (still) flowable mixture (I)might be poured into a mold with the desired shape. Herein, layerthicknesses of the flowable polysaccharide composition, for example thealginate composition of up to 50 cm are possible. Preferred shapes arebox shapes with a rectangular layout. Pouring can take place at anysuitable stage of the process.

Properties of the Porous Materials and Use

The present invention also relates to a porous material, which isobtained or obtainable by the process as described above. The porousmaterials of the present invention are preferably aerogels or xerogels.

For the purposes of the present invention, a xerogel is a porousmaterial which has been produced by a sol-gel process in which theliquid phase has been removed from the gel by drying below the criticaltemperature and below the critical pressure of the liquid phase(“subcritical conditions”). An aerogel is a porous material which hasbeen produced by a sol-gel process in which the liquid phase has beenremoved from the gel under supercritical conditions.

The process as disclosed above results in porous materials with improvedproperties, in particular improved thermal conductivity. Aerogelsproduced according to the process of the present invention preferablyhave a low density, and preferably high specific surface area, forexample in the range of from 200 to 800 m²/g. Furthermore, a pore volumein the range of from 2.1 to 9.5 cm³/g for pore sizes <150 nm can beobtained and preferably low thermal conductivity, for example as low as18±2 mW/m·K.

Furthermore, the present invention therefore is directed to a porousmaterial which is obtained or obtainable by the process for preparing aporous material as disclosed above. In particular, the present inventionis directed to a porous material which is obtained or obtainable by theprocess for preparing a porous material as disclosed above, wherein thedrying according to step c) is carried out under supercriticalconditions.

The porous material according to the invention preferably has a densityin the range of 0.005 to 1 g/cm³, preferably from 0.01 to 0.5 g/cm³(determined according to DIN 53420).

The average pore diameter is determined by scanning electron microscopyand subsequent image analysis using a statistically significant numberof pores. Corresponding methods are known to those skilled in the art.For characterization of the porous structure of aerogels a Nova 3000Surface Area Analyzer from Quantachrome Instruments was used. It usesadsorption and desorption of nitrogen at a constant temperature of −196°C.

The volume average pore diameter of the porous material is preferablynot more than 4 microns. The volume average pore diameter of the porousmaterial is particularly preferably not more than 3 microns, veryparticularly preferably not more than 2 microns and in particular notmore than 1 micron.

Although a very small pore size combined with a high porosity isdesirable from the point of view of a low thermal conductivity, from thepoint of view of production and to obtain a sufficiently mechanicallystable porous material, there is a practical lower limit to the volumeaverage pore diameter. In general, the volume average pore diameter isat least 20 nm, preferably at least 50 nm.

The porous material which can be obtained according to the inventionpreferably has a porosity of at least 70% by volume, in particular from70 to 99% by volume, particularly preferably at least 80% by volume,very particularly preferably at least 85% by volume, in particular from85 to 95% by volume. The porosity in % by volume means that thespecified proportion of the total volume of the porous materialcomprises pores. Although a very high porosity is usually desirable fromthe point of view of a minimal thermal conductivity, an upper limit isimposed on the porosity by the mechanical properties and theprocessability of the porous material.

The process of the invention gives a coherent porous material and notonly a polymer powder or particles. Here, the three-dimensional shape ofthe resulting porous material is determined by the shape of the gelwhich is in turn determined by the shape of the gelling apparatus. Thus,for example, a cylindrical gelling vessel usually gives an approximatelycylindrical gel which can then be dried to give a porous material havinga cylindrical shape. However, it is also possible to obtain a porousmaterial in the form of particles with a diameter of less than 500 μmaccording to the present invention as set out above.

The porous materials which can be obtained according to the inventionpreferably have a low thermal conductivity, a high porosity and a lowdensity combined with high mechanical stability. In addition, the porousmaterials preferably have a small average pore size. The combination ofthe abovementioned properties allows the materials to be used asinsulation material in the field of thermal insulation, in particularfor applications in the ventilated state as building materials.

The porous materials which can be obtained according to the inventionhave advantageous thermal properties and preferably also furtheradvantageous properties such as simple processability and highmechanical stability, for example low brittleness.

The present invention is also directed to the use of porous materials asdisclosed above or a porous material obtained or obtainable according toa process as disclosed above as thermal insulation material or forvacuum insulation panels. The thermal insulation material is for exampleinsulation material which is used for insulation in the interior or theexterior of a building. The porous material according to the presentinvention can advantageously be used in thermal insulation systems suchas for example composite materials.

According to a further embodiment, the present invention therefore isdirected to the use of porous materials as disclosed above, wherein theporous material is used in interior or exterior thermal insulationsystems.

Furthermore, the present invention relates to the use of the porousmaterials according to the invention for cosmetic applications, forbiomedical applications, for pharmaceutical applications and also forthe manufacture of a medical product. Such medical products include, forexample, wound dressings, transdermal dressings, wound plasters,implants, substrates for cultivating cells, means for the controlled, inparticular retarded, administering of active substances in the form ofsaid implants, but also as a preparation to effect such retardation thatcan be administered orally, or as so-called satiation comprimates thathave a satiation effect because of the expansion of the compressedporous shaped article in the stomach. The latter may also be loaded withdietary supplements, vitamins, minerals or other active substances.

According to a further aspect, the present invention relates to the useof porous materials as disclosed above or a porous material obtained orobtainable by the process as disclosed above as thermal insulationmaterial, for cosmetic applications, for biomedical applications or forpharmaceutical applications.

According to a further aspect, the present invention is also directed toa process for preparing a hydrogel, at least comprising the steps of:

a) providing a mixture (I) comprising

-   -   (i) a water soluble polysaccharide,    -   (ii) at least one compound suitable to react as cross-linker for        the polysaccharide or to release a cross-linker for the        polysaccharide,    -   (iii) water,        b) preparing a gel (A) comprising steps b1) and b2)    -   b1) exposing mixture (I) to carbon dioxide at a pressure in the        range of from 20 to 100 bar for a time sufficient to form a gel        (A), and    -   b2) depressurizing the gel (A).

In particular, the present invention is directed to a process forpreparing a hydrogel, at least comprising the steps of:

a) providing a mixture (I) comprising

-   -   (i) a water soluble alginate,    -   (ii) at least one compound suitable to react as cross-linker for        the alginate or to release a cross-linker for the alginate,    -   (iii) water,        b) preparing a gel (A) comprising steps b1) and b2)    -   b1) exposing mixture (I) to carbon dioxide at a pressure in the        range of from 20 to 100 bar for a time sufficient to form a gel        (A), and    -   b2) depressurizing the gel (A).

With respect to preferred embodiments of the process, reference is madeto the disclosure above regarding the process for preparing a porousmaterial according to the present invention.

Furthermore, the present invention is directed to a hydrogel, which isobtained or obtainable by the process as disclosed above. The presentinvention is also directed to the use of a hydrogel as disclosed abovefor cosmetic applications, for biomedical applications or forpharmaceutical applications.

The method according to the present invention allows to producehydrogels with polysaccharide concentrations, for example alginateconcentrations, as low as 0.05 wt %, being stable at ambient conditionsand can be stored in pure water for weeks without visible degradation.

The hydrogels prepared according to the process of the present inventionhave a stable structure and might be prepared in a thickness which issufficient for many cosmetic, biomedical or pharmaceutical applications.Furthermore, it is possible to entrap active components or adjuvants inthe gel structure using the process of the present invention. Gels witha uniform porosity might be obtained.

Preferred embodiments may be found in the claims and the description.Combinations of preferred embodiments do not go outside the scope of thepresent invention. Preferred embodiments of the components used aredescribed below.

The present invention includes the following embodiments, wherein theseinclude the specific combinations of embodiments as indicated by therespective interdependencies defined therein.

-   1. Process for preparing a porous material, at least comprising the    steps of:    -   a) providing a mixture (I) comprising        -   (i) a water soluble polysaccharide,        -   (ii) at least one compound suitable to react as cross-linker            for the polysaccharide or to release a cross-linker for the            polysaccharide,        -   (iii) water,    -   b) preparing a gel (A) comprising steps b1) and b2)    -   b1) exposing mixture (I) to carbon dioxide at a pressure in the        range of from 20 to 100 bar for a time sufficient to form a gel        (A), and    -   b2) depressurizing the gel (A),    -   c) exposing the gel (A) obtained in step b) to a water miscible        solvent (L) to obtain a gel (B),    -   d) drying of the gel (B) obtained in step c).-   2. The process according to embodiment 1, wherein the water soluble    polysaccharide is an alginate.-   3. The process according to any of embodiments 1 or 2, wherein    step b) is carried out at a temperature in the range of from 10 to    40° C.-   4. The process according to any of embodiments 1 to 3, wherein    carbon dioxide is gaseous under the reaction conditions in step b1).-   5. The process according to any of embodiments 1 to 4, wherein the    compound suitable to react as cross-linker for the polysaccharide or    to release a cross-linker for the polysaccharide is selected from    the group consisting of carbonates and hydroxy carbonates.-   6. The process according to any of embodiments 1 to 5, wherein    mixture (I) comprises calcium carbonate as compound (ii).-   7. The process according to any of embodiments 1 to 6, wherein the    solvent (L) is selected from the group consisting of ethers, esters,    alcohols, ketones, aldehydes, alkyl alkanoates, organic carbonates,    amides, sulfoxides, aliphatic and cycloaliphatic halogenated    hydrocarbons, halogenated aromatic compounds and fluorine-containing    ethers.-   8. The process according to any of embodiments 1 to 7, wherein the    solvent (L) used in step c) is selected from the group consisting of    C1 to C6 alcohols and C1 to C6 ketones and mixtures thereof.-   9. The process according to any of embodiments 1 to 8, wherein a    water insoluble solid (S) is added to mixture (I).-   10. The process according to any of embodiments 1 to 9, wherein a    compound (C) is added to mixture (I) in step a) which is suitable to    form a gel.-   11. The process according to any of claims 1 to 10, wherein step c)    is carried out at a pressure in the range of from 10 to 100 bar.-   12. The process according to any of embodiments 1 to 11, wherein the    drying according to step d) is carried out by converting the liquid    comprised in the gel into the gaseous state at a temperature and a    pressure below the critical temperature and the critical pressure of    the liquid comprised in the gel.-   13. The process according to any of embodiments 1 to 11, wherein the    drying according to step d) is carried out under supercritical    conditions.-   14. A porous material, which is obtained or obtainable by the    process according to any of embodiments 1 to 13.-   15. The use of porous materials according to embodiment 14 or a    porous material obtained or obtainable by the process according to    any of embodiments 1 to 13 as thermal insulation material, for    cosmetic applications, for biomedical applications or for    pharmaceutical applications.-   16. Process for preparing a porous material, at least comprising the    steps of:    -   a) providing a mixture (I) comprising        -   (i) a water soluble alginate,        -   (ii) at least one compound suitable to react as cross-linker            for the alginate or to release a cross-linker for the            alginate,        -   (iii) water,    -   b) preparing a gel (A) comprising steps b1) and b2)    -   b1) exposing mixture (I) to carbon dioxide at a pressure in the        range of from 20 to 100 bar for a time sufficient to form a gel        (A), and    -   b2) depressurizing the gel (A),    -   c) exposing the gel (A) obtained in step b) to a water miscible        solvent (L) to obtain a gel (B),    -   d) drying of the gel (B) obtained in step c).-   17. The process according to embodiment 16, wherein step b) is    carried out at a temperature in the range of from 10 to 40° C.-   18. The process according to any of embodiments 16 or 17, wherein    carbon dioxide is gaseous under the reaction conditions in step b1).-   19. The process according to any of embodiments 16 to 18, wherein    the compound suitable to react as cross-linker for the alginate or    to release a cross-linker for the alginate is selected from the    group consisting of carbonates and hydroxy carbonates.-   20. The process according to any of embodiments 16 to 19, wherein    mixture (I) comprises calcium carbonate as compound (ii).-   21. The process according to any of embodiments 16 to 20, wherein    the solvent (L) is selected from the group consisting of ethers,    esters, alcohols, ketones, aldehydes, alkyl alkanoates, organic    carbonates, amides, sulfoxides, aliphatic and cycloaliphatic    halogenated hydrocarbons, halogenated aromatic compounds and    fluorine-containing ethers.-   22. The process according to any of embodiments 16 to 21, wherein    the solvent (L) used in step c) is selected from the group    consisting of C1 to C6 alcohols and C1 to C6 ketones and mixtures    thereof.-   23. The process according to any of embodiments 16 to 22, wherein a    water insoluble solid (S) is added to mixture (I).-   24. The process according to any of embodiments 16 to 23, wherein a    compound (C) is added to mixture (I) in step a) which is suitable to    form a gel.-   25. The process according to any of claims 16 to 24, wherein step c)    is carried out at a pressure in the range of from 10 to 100 bar.-   26. The process according to any of embodiments 16 to 25, wherein    the drying according to step d) is carried out by converting the    liquid comprised in the gel into the gaseous state at a temperature    and a pressure below the critical temperature and the critical    pressure of the liquid comprised in the gel.-   27. The process according to any of embodiments 16 to 25, wherein    the drying according to step d) is carried out under supercritical    conditions.-   28. A porous material, which is obtained or obtainable by the    process according to any of embodiments 16 to 27.-   29. The use of porous materials according to embodiment 28 or a    porous material obtained or obtainable by the process according to    any of embodiments 16 to 27 as thermal insulation material, for    cosmetic applications, for biomedical applications or for    pharmaceutical applications.-   30. Process for preparing a hydrogel, at least comprising the steps    of:    -   a) providing a mixture (I) comprising        -   (i) a water soluble polysaccharide,        -   (ii) at least one compound suitable to react as cross-linker            for the polysaccharide or to release a cross-linker for the            polysaccharide,        -   (iii) water,    -   b) preparing a gel (A) comprising steps b1) and b2)    -   b1) exposing mixture (I) to carbon dioxide at a pressure in the        range of from 20 to 100 bar for a time sufficient to form a gel        (A), and    -   b2) depressurizing the gel (A).-   31. Process for preparing a hydrogel, at least comprising the steps    of:    -   a) providing a mixture (I) comprising        -   (i) a water soluble alginate,        -   (ii) at least one compound suitable to react as cross-linker            for the alginate or to release a cross-linker for the            alginate,        -   (iii) water,    -   b) preparing a gel (A) comprising steps b1) and b2)    -   b1) exposing mixture (I) to carbon dioxide at a pressure in the        range of from 20 to 100 bar for a time sufficient to form a gel        (A), and    -   b2) depressurizing the gel (A).-   32. The process according to any of embodiments 30 or 31, wherein    step b) is carried out at a temperature in the range of from 10 to    40° C.-   33. The process according to any of embodiments 30 to 32, wherein    carbon dioxide is gaseous under the reaction conditions in step b1).-   34. The process according to any of embodiments 30 to 33, wherein    the compound suitable to react as cross-linker for the    polysaccharide or to release a cross-linker for the polysaccharide    is selected from the group consisting of carbonates and hydroxy    carbonates.-   35. The process according to any of embodiments 30 to 34, wherein    mixture (I) comprises calcium carbonate as compound (ii).-   36. The process according to any of embodiments 30 to 35, wherein a    water insoluble solid (S) is added to mixture (I).-   37. The process according to any of embodiments 30 to 36, wherein a    compound (C) is added to mixture (I) in step a) which is suitable to    form a gel.-   38. A hydrogel, which is obtained or obtainable by the process    according to any of embodiments 30 to 37.-   39. The use of a hydrogel according to embodiment 38 or a hydrogel    obtained or obtainable by the process according to any of    embodiments 30 to 37 for cosmetic applications, for biomedical    applications or for pharmaceutical applications.

Examples will be used below to illustrate the invention.

EXAMPLES 1. Example 1

Sodium alginate solution was prepared by gentle stirring of sodiumalginate powder (obtained from Sigma-Aldrich) with appropriate amount ofwater for 12 h. Calcium carbonate was suspended in water by vigorousmixing for 5 min. Keeping the agitation up, a certain part of thesuspension was bled off and immediately transferred into the sodiumalginate solution (0.25; 0.5; 1.0 wt %) to reach a target sodiumalginate/CaCO₃ ratio (Table 1). The mixture was again agitated until itbecame homogeneous. All prepared suspensions were transferred into ahigh pressure autoclave for subsequent gelation. Final concentrations ofsodium alginate solutions are listed in Table 1. Sodium alginate/CaCO₃ratio of 1:0.1825 denoted as factor F=1.0.

The autoclave was pressurized with gaseous carbon dioxide up to 50 barat room temperature (25° C.). Pressure was maintained for 12 h and thenslowly released (0.2 bar/min). Hydrogels formed were either transparentor translucent. The gels were washed with water and successivelyimmersed in ethanol/water mixtures with concentrations of 30, 60, 90 and100 wt % for 12 h in each. Step wise concentration is recommended ashigh concentration gradients during solvent exchange cause irreversibleshrinkage to the hydrogel.

Alcogels were packed into filter paper parcels, placed into preheatedhigh pressure autoclave (40° C.) and filled with ethanol to preventpremature solvent evaporation. Supercritical drying was performed usingthe same autoclave as for gelation. The autoclave was sealed and CO₂ wasfilled in by a membrane pump. Keeping the pressure constant around 120bar, 6-7 residence volumes of CO₂ was used to dry the gel. Then systemwas depressurized in 1 h followed by cooling down to room temperature.Properties of the resulting aerogels are summarized in Table 1.

Alternative with high pressure solvent exchange—The gels were washedunder increased pressure of CO₂ of 50 bar at room temperature with waterand successively immersed in ethanol/water mixtures with concentrationsof 30, 60, 90 and 100 wt % for 12 h in each. Step wise concentration isrecommended as high concentration gradients during solvent exchangecause irreversible shrinkage to the hydrogel

TABLE 1 Properties of the aerogels from Example 1 Final sodium alginateBulk BET sur- BJH pore Thermal concentration, density, face area,volume, conductivity, wt % F g/cm³ m²/g cm³/g mW/m · K ¹⁾ 0.49 0.5 0.037473 ± 90 5.68 0.25 1.0 0.024 436 ± 96 4.34 0.49 1.0 0.028 479 ± 67 6.9822 ± 2 0.97 1.0 0.042 487 ± 67 4.55 0.25 2.0 — — — 18 ± 2 0.50 2.0 — — —19 ± 2 ¹⁾ Thermal conductivity was determined by hot-wire measurementsat ambient pressure and room temperature. Procedure as described byReichenauer et. al, (Reichenauer, G., Heinemann, U., and Ebert, H.-P.(2007). Relationship between pore size and the gas pressure dependenceof the gaseous thermal conductivity. Colloids and Surfaces A:Physicochemical and Engineering Aspects 300, 204-210).

For characterization of the porous structure of aerogels a Nova 3000Surface Area Analyzer from Quantachrome Instruments was used. It usesadsorption and desorption of nitrogen at a constant temperature of −196°C.

According to the present invention, CaCO₃ is directly dispersed inwater, which is then slowly dissolved by decreasing the pH value.Solubility of carbon dioxide increases with rising pressure along withlowering of pH down to 3. Solubility of calcium carbonate also increaseswith pressure resulting in release of calcium ions. At conditions usedin this study for gelation (50 bar, 25° C.), solubility of CaCO₃ is muchhigher (2.9 g/L) than at ambient conditions (0.01 g/L). Concentration ofCaCO₃ in the final mixtures lies between 0.23 and 4.4 g/L. Hence morethan half of introduced calcium carbonate is dissolved at equilibriumconditions and available to crosslink alginate chains. The hydrogelsobtained using the disclosed CO₂ induced gelation were stable at ambientconditions and can be stored in pure water for weeks without visibledegradation (in presence of a preservative to avoid bacterialdecomposition).

The samples prepared by the process disclosed herein show extremely highBJH pore volumes of up to 5.68 and 6.98 cm³/g, respectively, displayingvalues close to silica aerogels (typically around 6 cm³/g).

2. Example 2

Sodium alginate of 3 wt % was mixed with 3 wt % water solution of thesecond polymer (Table 2). In cases of base soluble mixtures such asLignin and Eudragit L100, 1 M sodium hydroxide was used instead water todissolve Eudragit L100 and lignin. To incorporate insoluble gel/aerogelmatrices, hydrophobic silica alcogels using MTMS were first preparedusing the recipe described in (Rao et al., 2006, Synthesis of flexiblesilica aerogels using methyltrimethoxysilane (MTMS) precursor. Journalof Colloid and Interface Science 300, 279-285). Then alcogels werewashed with water to obtain hydrogel. The later was crushed anddispersed in sodium alginate solution. Mixture was diluted with water toreach the overall polymer concentration of 1.5 wt %. The subsequentprocedure was the same as described in Example 1, with the onlydifference that solid calcium carbonate was dispersed in the mixtureinstead of using suspended CaCO₃.

TABLE 2 Properties of hybrid alginate-based aerogels Bulk BET sur- BJHpore density, face area, volume, Polymer F g/cm³ m²/g cm³/gCarboxymethyl 2.0 0.025 812 7.90 cellulose Gellan gum 2.0 0.033 346 2.98Eudragit L100 1.0 0.039 — — Gelatin 1.0 0.043 208 3.24 Polyvinylalcohol1.0 0.038 690 5.47 Pluronic 1.0 0.051 444 7.93 Starch ¹⁾ 1.0 0.066 5446.77 Starch ¹⁾ 2.0 0.058 435 4.08 Polyethylene glycol 1.0 0.054 555 6.96(Mw = 10 000) Polyethylene glycol 1.0 0.068 591 6.94 (Mw = 100 000)Methylcellulose 1.0 0.058 505 9.50 Lignin ²⁾ 2.0 0.062 456 2.6  AmidatedPectin ³⁾ 1.0 0.099 739 7.88 h-Carrageenan ³⁾ 1.0 0.049 593 4.77Hydrophobic silica 1.0 0.201 436 2.11 dispersed in alginate ³⁾ ¹⁾alginate/starch ratio of 5.0 (g/g) was used, ²⁾ alginate/lignin ratio of4.0 (g/g) was used ³⁾ mixture of alginate (3 wt %) and second biopolymer(3 wt %) not diluted further to 1.5%.

The thermal conductivities of select biopolymer mixtures (overallcomposition: 1.5 wt %) were analyzed and are presented in Table 3. Themethod of measurement is by hot-wire measurements as described inReichenauer et. al, (Reichenauer, G., Heinemann, U., and Ebert, H.-P.(2007). Relationship between pore size and the gas pressure dependenceof the gaseous thermal conductivity. Colloids and Surfaces A:Physicochemical and Engineering Aspects 300, 204-210).

TABLE 3 Thermal conductivity of hybrid alginate-based aerogels WeightThermal ratio conduc- (Alginate/ Cross- tivity, Second second linkingBulk Dimensions mW/ Bio- Bio- factor density, (mm) m · polymer polymer)(F) g/cm³ l w h K ¹⁾ Starch 1.0 1.0 0.100 77.6 34.6 6.9 21.6 Starch 1.02.0 0.067 76.5 34.6 9.5 20.4 Lignin 3.0 1.0 0.048 51.3 34.7 7.3 19.4 ¹⁾Thermal conductivity was determined by hot-wire measurements at ambientpressure and room temperature

3. Example 3

Powder of zinc hydroxy carbonate, nickel hydroxy carbonate or cobaltcarbonate was added to sodium alginate solution of 3 wt %. The mixturewas vortexed for 1 min. The subsequent procedure was the same asdescribed in Example 1. Textural properties are listed in Table 4.

TABLE 4 Properties of alginate-based aerogels with various cations BulkBET sur- BJH pore Crosslinking density, face area, volume, cation Factor¹⁾ g/cm³ m²/g cm³/g Zinc (ZIN-D001) 1.0 0.093 553 5.23 Cobalt (COB-D001)1.0 0.098 546 6.36 Nickel (NIC-D001) 1.0 0.100 668 6.40 ¹⁾ molar amountof cations was the same as for calcium in example 1

1. A porous material, which is obtained or obtainable by the processcomprising: a) providing a mixture (I) comprising (i) a water solublepolysaccharide, (ii) a compound which reacts as a cross-linker for thepolysaccharide or which releases a cross-linker for the polysaccharide,and (iii) water; b) preparing a gel (A) by a process comprising b1)exposing the mixture (I) to carbon dioxide at a pressure in a range offrom 20 to 100 bar for a time sufficient to form a gel (A), and b2)depressurizing the gel (A); c) exposing the gel (A) to a water misciblesolvent (L) to obtain a gel (B); and d) drying of the gel (B).
 2. Theporous material of claim 1, wherein the water soluble polysaccharide isselected from the group consisting of agar, alginate, carrageenan,cellulose, hyaluronic acid, pectin, starch, xanthan gum, modifiedcellulose, chitin, chitosan, and a mixture thereof.
 3. The porousmaterial of claim 1, wherein the water soluble polysaccharide is analginate.
 4. The porous material of claim 1, wherein the preparing in b)is carried out at a temperature of from 10 to 40° C.
 5. The porousmaterial of claim 1, wherein carbon dioxide in the exposing in b1) isgaseous.
 6. The porous material of claim 1, wherein the compound whichreacts as cross-linker for the polysaccharide or which releases across-linker for the polysaccharide is selected from the groupconsisting of a carbonate, a hydroxy carbonate, and a mixture thereof.7. The porous material of claim 1, wherein the compound which reacts ascross-linker for the polysaccharide or which releases a cross-linker forthe polysaccharide in the mixture (I) comprises calcium carbonate. 8.The porous material of claim 1, wherein the solvent (L) is selected fromthe group consisting of an ether, an ester, an alcohol, a ketone, analdehyde, an alkyl alkanoate, an amide, a sulfoxide, an organiccarbonate, an aliphatic and cycloaliphatic halogenated hydrocarbon, ahalogenated aromatic compound, a fluorine-containing ether, and amixture thereof.
 9. The porous material claim 1, wherein the solvent (L)is selected from the group consisting of a C1 to C6 alcohol, a C1 to C6ketone, and a mixture thereof.
 10. The porous material of claim 1,wherein a water insoluble solid (S) is added to the mixture (I).
 11. Theporous material of claim 1, wherein a compound (C) is added to themixture (I) in the providing in a), wherein the compound (C) is selectedfrom the group consisting of a natural hydrocolloid forming polymer, asynthetic hydrocolloid-forming polymer, and a mixture thereof.
 12. Theporous material of claim 1, wherein the exposing in c) is carried out ata pressure in a range of from 10 to 100 bar.
 13. The porous materialaccording to claim 1, wherein the drying in d) is carried out byconverting a liquid in the gel (B) into a gaseous state at a temperatureand a pressure below the critical temperature and the critical pressureof the liquid in the gel (B).
 14. The porous material according to claim1, wherein the drying in d) is carried out under supercriticalconditions.
 15. A cosmetic, biomedical, or pharmaceutical product,comprising: a thermal insulation material comprising the porous materialof claim 1.